There is known a motorcycle of the type wherein a drive source such as an engine and a power transmission mechanism such as a clutch and continuously variable transmission are accommodated within a single casing and are mounted as a power unit to a vehicle body frame. Since various devices which generate heat with operation of clutch and transmission are accommodated within the casing of the power unit, it is necessary to cool the interior of the casing efficiently. Therefore, many of power units of this type are provided with a cooling device wherein air is introduced from the exterior of the casing and is allowed to flow through the interior of the casing to effect heat exchange.
A conventional cooling device is known wherein a centrifugal fan is mounted to a driving pulley in a belt type continuously variable transmission, air introduced from an intake port of a casing is fed into the casing by a radially outward blow-off action of the centrifugal fan, and heat-generating rotating devices such as continuously variable transmission and clutch are cooled by a flow of the air (see, for example, Japanese Patent Laid Open No. Sho 58-109762).
However, according to the structure of the above conventional cooling device, the introduced air is allowed to flow through the interior of the casing by the radial blow-off action of the centrifugal fan, so when a heat-generating rotating device such as clutch is spaced very far from a rotary shaft of the centrifugal fan, it is difficult to cool the device efficiently. That is, when a heat-generating rotating device to be cooled is spaced very far from a rotary shaft of the centrifugal fan, the introduced air is warmed by another heat-generating device while flowing through the interior of the casing and its heat-exchanging capacity is deteriorated upon arrival at the desired heat-generating device. Further, the centrifugal fan, due to characteristics thereof, cannot perform direct sending of air even when disposed coaxially with a heat-generating rotating device. In using the above conventional cooling device, therefore, it is required in many cases that the centrifugal fan and the heat-generating rotating device to be cooled be spaced away from each other.
Accordingly, it is an object of the present invention to provide a power unit cooling device capable of efficiently cooling a heat-generating rotating device to be cooled which is disposed within a casing.